The present invention relates to a device for driving a spindle mounting slide.
The invention relates in particular to a device for driving the spindle mounting slide in a machine tool supported by a frame and controlled by linear motors that drive the tool at high speed along one or more axes.
In modern automatic machine tools, the tool is usually mounted on a supporting element driven by linear motors along three axes which are perpendicular to each other.
Automatic machine tools known in prior art comprise a linear electric motor which provides at least the vertical drive for the spindle mounting slide. The linear electric motor comprises a rotor, also known as primary member or simply xe2x80x9cprimaryxe2x80x9d and a stator, also known as secondary member or simply xe2x80x9csecondaryxe2x80x9d, the primary being integral with the spindle mounting slide, and the secondary being integral with the vertical slideway in which the slide runs.
The vertical slideway has, symmetrically positioned at the bottom of it, two pairs of fixed linear guides, for example of the recirculating ball type, which are permanently engaged with two corresponding longitudinal runners made on the mobile slide. The fixed guides also keep the primary and the secondary at a fixed distance from each other, this distance constituting the air gap of the linear motor.
The fixed guides are positioned on the vertical slideway at a suitable axial distance from each other to form on the slideway itself a particularly rigid zone that is capable of easily resisting the backlash transmitted by the spindle to the slide during machining.
The longitudinal extension of the slide is greater than the axial distance between the two pairs of fixed guides and the distance between the two pairs of guides is approximately equal to the vertical travel of the mobile slide. Indeed, lengthening the longitudinal extension of the fixed guides would unduly encumber the machine tool, with negative repercussions on the accuracy and performance of the machine.
Adding more pairs of fixed guides, although it does not considerably increase the weight of the slideway, nevertheless makes it necessary to lengthen the longitudinal runners in order to make the coupling between runner and guide permanent. If the coupling were not permanent, there would be a serious risk of dangerous collisions with the ends of the runners when they enter the guides on account of the high speed at which the spindle mounting slide moves.
For obvious practical purposes, the primary of the linear motor, which is mounted close to the upper part of the mobile slide, occupies a longitudinal portion of the slide itself so that it permanently faces the secondary which is fixed to the vertical slideway.
When the spindle mounting slide is driven to its uppermost position, the part of the slide with the primary on it projects with respect to the support offered to it by the fixed sliding guides and, consequently, the considerable magnetic force between the primary and the secondary attracts the primary and the secondary to each other. This tends to bend the slide and to move it out of alignment relative to its fixed support, with negative repercussions on machining precision.
The aim of the present invention is to overcome the above mentioned disadvantage by providing a device that is used to drive the spindle mounting slide and that is light, compact and economical to construct.
The present invention accordingly provides a machine tool comprising at least one spindle mounting slide extending mainly longitudinally along a defined axis and able to move in a direction parallel to said axis at least between a first retracted end position and a second extended end position relative to a fixed slideway extending in said direction, driven by at least one linear electric motor, a primary member of which is connected to the slide and a secondary member of which is connected to the slideway, said slideway comprising fixed sliding guides which are integral with the slideway itself, extend longitudinally in said direction for a distance shorter than the longitudinal extension of the secondary member and engage with runners that are integral with the slide, a longitudinal portion of the spindle mounting slide comprising at least one portion of the primary member which projects with respect to the fixed sliding guides at at least one of the two end positions of the slide itself, wherein the slide comprises sliding contact means that oppose the slideway at least when the portion of the primary member is in the position where it projects with respect to the fixed sliding means and the magnetic force is attracting the primary member and the secondary member of the linear electric motor to each other.